Officers keeping an “open mind” about whether the rock was left there deliberately, but say a pattern has developed and they are investigating the matter.

In the latest incident, mechanic Sion Jones, 22, was driving home from work on a straight stretch of the road when his Volkswagen Golf hit the rock, the size of two bricks, opposite Pont Plas Newydd, at about 6.45pm on Tuesday night.

He suffered arm burns from his air bag and paramedics took him for treatment to Ysbyty Gwynedd in Bangor.

He said: “It was dark and raining. There was a big rock in the road and I didn’t see it until the last minute because there was a car coming towards me with its lights on. I hit the rock, my car went up in the air and my air bags deployed.”

He added: “The rock went into the sump in the engine and the floor panel went up. The engine’s gone. There was even smoke in the car.”

Damage after a car collided with a rock in the road on the A499.

Mr Jones managed to pull over. He lives nearby in Llandwrog and his mum dialled 999.

“My arm was cut from the burns from the air bag and I had to get it glued.”

Mr Jones warned about the danger of the rocks and the stupidity of the act if it was being done deliberately.

He said: "You can get to a massive speed of 60mph or 70mph on that stretch. Someone’s going to be killed.”

PC Trystan Bevan of the North Wales Police Western Roads Policing Unit said: “This is the third similar incident in the surrounding area where a rock has been left in the middle of the road.

“At present we are keeping an open mind as to whether the rock was deliberately left in the road or if it has fallen off a back of a truck. However large grass verges are on either side of the road and the rock is now being examined as part of our investigation.

He warned: “This particular stretch is a 60mph area and is unlit so this could easily have resulted in a serious collision. Thankfully the man sustained minor injuries however the car has been written off.

Further patrols of the area are being undertaken and police are appealing to anybody who may have information, or anybody who may have witnessed suspicious behaviour in the area to contact the Llandygai Roads Policing Unit on 101.